Convertisseurs élévateurs pour lasers à fibre : transition à tension nulle

Convertisseurs élévateurs pour lasers à fibre : transition à tension nulle

Abstract

Boost stages in fiber laser pump sources step 200 V to 400 V up to 600 V to 800 V, and turn-on loss limits their switching frequency. This article gives the ZVT sequence, the resonant network equations for a 1 nF, 25 µH, 200 kHz design, four field pain points, and five steps that reach 97.5% efficiency with a stable transition time.

1. Introduction

A pump source that runs from a 200 V to 400 V DC bus needs a boost stage to reach 600 V to 800 V for a high-voltage diode array or for the DC link of a downstream LLC converter. Hard-switched boost stages lose most of their efficiency at that voltage level.

Turn-on loss alone caps the practical frequency near 150 kHz, which forces large inductors and bulk capacitors. The loss also radiates noise into the laser diode current loop, so the gain in efficiency and the gain in emission margin pull in the same direction.

Zero-voltage transition applies to a conventional boost without changing the converter architecture. An auxiliary branch with a small inductor and an auxiliary switch discharges the main switch output capacitance before the main switch turns on.

This article covers the six-stage ZVT sequence, the resonant network arithmetic, four field pain points, and five design steps built on N4700 Class I ceramic capacitors.

2. Mécanisme de base

2.1 Turn-on loss in a hard-switched boost

The main switch turns on while its drain still sits at the output voltage, so the transition dissipates:

P_sw_on = ½ × V_out × I_in × t_rise × f_sw

where P_sw_on is the turn-on loss, V_out the output voltage, I_in the input current, t_rise the voltage fall time, and f_sw the switching frequency. A 600 V output at 20 A with a 40 ns rise and 150 kHz of switching gives 36 W in one device.

2.2 The ZVT sequence

The auxiliary branch turns on first and ramps current through the resonant inductor at V_out / L_r. When that current exceeds the input current, the resonant capacitor discharges through the auxiliary loop instead of through the main device.

Stage by stage: the auxiliary switch turns on; the resonant current overtakes the input current; C_r discharges to zero and the main body diode conducts; the main switch turns on at zero volts and the auxiliary switch turns off; the inductor energy transfers to the output; the main switch turns off and C_r charges at a controlled dV/dt.

2.3 Resonant network sizing

The transition time follows the resonant product of the auxiliary branch:

t_zvt = π × √(L_r × C_r)

where t_zvt is the time from auxiliary turn-on to main turn-on, L_r the resonant inductance, and C_r the total resonant capacitance, which includes the output capacitance C_oss of the main switch and any external capacitor.

At 200 kHz the period is 5 µs, so a 500 ns transition uses 10% of it. With C_r = 1 nF, built from 300 pF of C_oss and 700 pF external, the product L_r × C_r must stay below 25.3 × 10⁻¹⁸ s², which allows L_r up to 25 µH.

The branch impedance then sets the peak current:

Z_r = √(L_r / C_r)

where Z_r is the characteristic impedance of the resonant branch. With 25 µH and 1 nF, Z_r is 158 Ω, so the peak resonant current is V_out / Z_r, or 3.8 A. That peak is 19% of a 20 A input current, and it must exceed the input current for the drain to reach zero volts.

2.4 Resonant capacitor loss

The capacitor carries a resonant current burst once per cycle:

I_C_rms = I_peak × √(t_zvt × f_sw / 2)

where I_C_rms is the root-mean-square resonant current, I_peak the peak current, t_zvt the transition time, and f_sw the switching frequency. The values of §2.3 give 0.85 A.

That current dissipates in the capacitor ESR:

P_C = I_C_rms² × ESR

where P_C is the capacitor dissipation and ESR the equivalent series resistance. At 100 mΩ of X7R ESR the loss is 72 mW; at the 15 mΩ of an N4700 part it is 11 mW. Across 16 capacitors in 8 modules the difference reaches 976 mW of heat inside the enclosure.

2.5 Loss after the transition is added

ZVT removes the turn-on term and leaves turn-off:

P_sw_ZVT = ½ × V_out × I_in × t_fall × f_sw

where P_sw_ZVT is the remaining switching loss and t_fall the current fall time. At 600 V, 20 A, a 20 ns fall, and 200 kHz, the figure is 24 W per MOSFET against 72 W for the hard-switched case, which lifts full-load efficiency from 94.2% to 97.5%.

Fig. 1 — The auxiliary branch discharges the 1 nF resonant capacitor before S1 turns on, cutting turn-on loss at 600 V and 200 kHz.

3. Points faibles en matière d'ingénierie

Pain Point 1: Resonant capacitor temperature drift moves the transition time

The transition time follows the resonant LC product, so any capacitance drift moves the moment the drain reaches zero volts. An X7R capacitor with ±15% capacitance over temperature shifts t_zvt by ±7.5%.

At 500 ns that shift is ±37 ns. The main switch then turns on while the drain still holds charge, the transition becomes partly hard-switched, and the stage emits a spike that the ZVT design was meant to remove.

Pain Point 2: Voltage rating too tight for an 800 V bus

In an 800 V boost, the resonant capacitor holds the full output voltage in the off state and sees 1.2× to 1.5× that value during a load step. A 630 V part has no margin left at that point.

A capacitor that runs near its rated voltage sustains partial discharge in the dielectric. Each event erodes the film and raises ESR, and the resonant current then heats the part further.

Pain Point 3: Parasitic inductance in the resonant loop

The loop inductance is L_r plus the capacitor ESL plus the trace inductance. At 25 µH of designed L_r, 50 nH of parasitics changes nothing at the resonant frequency.

At the 1 MHz to 10 MHz switching harmonics it matters. The parasitic elements form a second resonance that overshoots the drain by 50 V to 100 V and pushes the device outside its safe operating area.

Pain Point 4: Auxiliary gate bypass capacitors under resonant stress

The auxiliary switch sees the same dV/dt as the main device, and its gate bypass capacitor also carries resonant current. An X7R part with high dielectric loss heats internally at the resonant frequency.

Micro-cracks form after 10,000 h to 15,000 h of continuous operation. The auxiliary branch then fails to complete the transition, the converter reverts to hard switching, and the switching loss doubles.

4. Étapes de conception

Strategy 1: Use Class I dielectric for the resonant capacitor

N4700 holds a temperature coefficient of ±30 ppm/°C, so capacitance stays within ±0.35% from −55 °C to +125 °C, against ±15% for X7R. On a 500 ns transition that limits the timing variation to ±1.75 ns.

Utilisez le N4700 parts at the resonant node. The same dielectric holds tan δ at 0.1%, so the 0.85 A resonant current costs 11 mW instead of 72 mW.

Strategy 2: Rate the resonant capacitor at 2× the output voltage

An 800 V boost uses 1.6 kV to 2 kV parts. The 100% margin keeps transient peaks below the partial discharge inception voltage, so the dielectric does not accumulate damage over the service life.

HVC N4700 parts from 1 kV to 100 kV hold PD below 5 pC at 1.5× rated voltage, which covers the clamp capacitor and the snubber positions in the same loop.

Strategy 3: Cut the resonant loop inductance

Route the resonant capacitor with short, wide traces of at least 3 mm on 1 oz copper and a total length below 15 mm. Place the part within 5 mm of the main drain terminal and use at least four vias for the ground return.

That layout holds total parasitic inductance below 10 nH and keeps the harmonic overshoot below 20 V. Measure the loop on the first article with a current probe before release, because a 50 nH mistake costs 50 V of overshoot.

Strategy 4: Use Class I bypass capacitors at the auxiliary gate

Placer 100 nF de HVCT8G capacitance across the auxiliary gate driver supply with ESL below 3 nH. The 0.1% dissipation factor removes the internal heating that cracks X7R parts.

Failure rate falls below 0.1 FIT against 1 FIT to 10 FIT for X7R under the same resonant stress, which removes the auxiliary-branch failure mode of Pain Point 4.

Strategy 5: Trim the auxiliary timing with load, and clamp the auxiliary drain

Adjust the auxiliary gate delay with the sensed input current, 10 ns to 50 ns per ampere of change, so the branch carries just enough current for zero-voltage switching from 10% to 100% load. Extra current only adds conduction loss in the auxiliary device.

Add a 1 kV N4700 capacitor of 1 nF to 10 nF in series with a diode à récupération rapide from the auxiliary drain to the output rail. The clamp holds that drain below 1.2× V_out, and its low ESR lets it respond within 5 ns. Where the boost feeds a resonant tank, a condensateur à film at the DC link absorbs the remaining step energy.

Paramètre de composant	Valeur typique	Condition de test
Tension nominale (série HVCT8G)	10 kV DC	haute tension 1.5× pendant 60 s
Capacitance	4700 pF ±20% (472M)	1 MHz, 1 V efficace
ESR à 1 MHz	inférieur à 50 mΩ	-
Inductance de boucle (accord E40)	en dessous de 8 nH	Espacement des bornes de 40 mm
La resistance d'isolement	supérieure à 10 GΩ à 500 V CC	25 °C, 60 % HR
Plage de température	−40 °C à +125 °C	Série IEC 60068-2
Capacité dv/dt d'impulsion	au-dessus de 50 kV/µs	à la tension nominale

5. HVC N4700 capacitors for ZVT boost networks

Three requirements drive this node: a capacitance that holds its value across temperature, low loss under a resonant current burst, and a voltage rating with margin on an 800 V bus. HVC builds N4700 (T3M) Class I parts on a SrTiO3 base for that set, in a 6,000 m² owned factory in Dongguan, from 1 kV to 100 kV.

Six paramètres importants ici

• Temperature coefficient of ±30 ppm/°C. Capacitance stays within ±0.35% from −55 °C to +125 °C, against ±15% for X7R, so the resonant frequency and the transition time hold within ±1.75 ns at 500 ns.
• tan δ of 0.1% at 1 MHz. On a 1 nF resonant capacitor at 600 V and 200 kHz the dielectric term drops to 0.72 W against 7.2 W for an X7R part at 2%, which lowers the load on the module heatsink.
• Partial discharge below 5 pC at 1.5× rated voltage, across a range from 1 kV to 100 kV. PD control removes the cumulative dielectric damage that raises ESR on a resonant node.
• Package inductance below 30 nH in the radial-lead HVCT8G outline, which keeps the resonant loop clean up to 1 MHz and holds harmonic overshoot on the drain below 20 V.
• No aging drift and no DC bias loss. Capacitance stays within 2% after 50,000 h at 85 °C, with intrinsic life beyond 100,000 h, so the transition time does not walk over a 10-year service life.
• Qualified as a drop-in for first-tier brands in field-repair programmes and specified by laser OEMs in new soft-switching platforms. HVC parts match the electrical parameters and package dimensions of Murata composants haute tension de type vis et à câbles radiaux.

Livraison et support

Standard parts ship in 4–8 weeks, and matched custom sets in 6–10 weeks. HVC applications engineering supports resonant network sizing, transition time analysis, capacitor thermal calculation, and layout review for minimum loop inductance.

Send the input and output voltages, switched current, switching frequency, C_oss of the main switch, and the target transition time. HVC returns a capacitor selection sheet with capacitance and ESR distribution plus a two-sample evaluation kit within five working days.

6. Questions fréquentes

Q1: How does ZVT cut turn-on loss in a boost converter?

An auxiliary branch ramps current through a resonant inductor until it exceeds the input current. The resonant capacitor then discharges to zero and the main body diode conducts, so the main switch turns on with no voltage across it. Turn-on loss falls from 36 W to zero at 600 V, 20 A, and 150 kHz, and total switching loss falls from 72 W to 24 W.

Q2: How do I size the resonant inductor and capacitor?

From the transition time: t_zvt = π × √(L_r × C_r). Keep the transition below 10% of the switching period, which is 500 ns at 200 kHz. With C_r = 1 nF, L_r may reach 25 µH, and the branch impedance √(L_r / C_r) is 158 Ω.

Q3: Why does the resonant capacitor temperature stability matter?

Because the transition time follows the LC product. An X7R part with ±15% capacitance over temperature shifts t_zvt by ±7.5%, or ±37 ns at 500 ns, and the main switch turns on with charge still on the drain. N4700 holds ±0.35% and limits the shift to ±1.75 ns.

Q4: What voltage rating does the resonant capacitor need?

Twice the maximum output voltage. An 800 V boost uses 1.6 kV to 2 kV parts, because load steps push the bus to 1.2× to 1.5× nominal. The margin keeps peaks below the partial discharge inception voltage, and HVC N4700 parts hold PD below 5 pC at 1.5× rated voltage.

Q5: How much loss does the resonant capacitor add?

P_C = I_C_rms² × ESR with I_C_rms = I_peak × √(t_zvt × f_sw / 2). At 3.8 A peak, 500 ns, and 200 kHz the RMS current is 0.85 A, so 100 mΩ of X7R ESR gives 72 mW against 11 mW at the 15 mΩ of N4700. Across 16 capacitors that is 976 mW of difference inside the enclosure.

Q6: What breaks the auxiliary branch in the field?

Its gate bypass capacitor. Resonant current heats a lossy X7R part internally, and micro-cracks appear after 10,000 h to 15,000 h. The branch then fails to complete the transition and the converter returns to hard switching. Class I parts below 0.1 FIT remove that mode.

7. Références

1. E. Kuffel, WS Zaengl, J. Kuffel, Principes fondamentaux de l'ingénierie haute tension, 2e éd., Newnes, 2000.
2. N. Mohan, TM Undeland, WP Robbins, Électronique de puissance : convertisseurs, applications et conception, 3e éd., Wiley, 2003.
3. Avis MIL-HDBK-217F 2, Prédiction de la fiabilité des équipements électroniques 1995.
4. CEI 60384-1:2018, Condensateurs fixes destinés aux équipements électroniques — Partie 1 : Spécifications générales.
5. AND9800/D — Sélection du condensateur d'amortissement pour les applications d'entraînement de moteur (Tableau 15 Données de test), onsemi, AND9800-D, https://m.onsemi.jp/pub/Collateral/AND9800-D.PDF .
6. Wook-Jin Lee et Seung-Ki Sul, Stabilisation de la tension du bus DC pour un onduleur à capacité de bus DC réduite, IEEE Transactions on Industry Applications, vol. 50, no. 1, pp. 404-414, 2014. https://doi.org/10.1109/tia.2013.2268733.
7. ГОСТ IEC 61071-2014, Condensateurs électrolytiques (Condensateurs pour l'électronique de puissance). Moscou : Standard, 2014.
8. D. Li, Y. Zhong, X. Zhou et G. Zhai, Méthode d'analyse de la rupture des condensateurs céramiques multicouches par le champ de phase, IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 14, no. 2, pp. 211-220, 2024. https://doi.org/10.1109/tcpmt.2024.3359598.
9. AN2001_06 — Connexion en série des IGBT, Infineon Technologies, AN2001_06, http://www.infineon.com/dgdl/Infineon-AN2001_06_Series_connection_of_IGBTs-AN-v1.0-en.pdf?fileId=db3a304412b407950112b40f000d12f7 .
10. Équipe d'ingénierie des applications HVC, Condensateur céramique haute tension Ressources techniques, https://www.hv-caps.com